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SYSCTL(3) Library Functions Manual SYSCTL(3) NAME sysctl, sysctlbyname, sysctlnametomib -- get or set system information LIBRARY Standard C Library (libc, -lc) SYNOPSIS #include <sys/sysctl.h> int sysctl(const int *name, u_int namelen, void *oldp, size_t *oldlenp, const void *newp, size_t newlen); int sysctlbyname(const char *name, void *oldp, size_t *oldlenp, const void *newp, size_t newlen); int sysctlnametomib(const char *name, int *mibp, size_t *sizep); DESCRIPTION The sysctl() function retrieves system information and allows processes with appropriate privileges to set system information. The information available from sysctl() consists of integers, strings, and tables. Infor- mation may be retrieved and set from the command interface using the sysctl(8) utility. Unless explicitly noted below, sysctl() returns a consistent snapshot of the data requested. Consistency is obtained by locking the destination buffer into memory so that the data may be copied out without blocking. Calls to sysctl() are serialized to avoid deadlock. The state is described using a ``Management Information Base'' (MIB) style name, listed in name, which is a namelen length array of integers. The sysctlbyname() function accepts an ASCII representation of the name and internally looks up the integer name vector. Apart from that, it behaves the same as the standard sysctl() function. The information is copied into the buffer specified by oldp. The size of the buffer is given by the location specified by oldlenp before the call, and that location gives the amount of data copied after a successful call and after a call that returns with the error code ENOMEM. If the amount of data available is greater than the size of the buffer supplied, the call supplies as much data as fits in the buffer provided and returns with the error code ENOMEM. If the old value is not desired, oldp and oldlenp should be set to NULL. The size of the available data can be determined by calling sysctl() with the NULL argument for oldp. The size of the available data will be re- turned in the location pointed to by oldlenp. For some operations, the amount of space may change often. For these operations, the system at- tempts to round up so that the returned size is large enough for a call to return the data shortly thereafter. To set a new value, newp is set to point to a buffer of length newlen from which the requested value is to be taken. If a new value is not to be set, newp should be set to NULL and newlen set to 0. The sysctlnametomib() function accepts an ASCII representation of the name, looks up the integer name vector, and returns the numeric representation in the mib array pointed to by mibp. The number of elements in the mib array is given by the location specified by sizep before the call, and that loca- tion gives the number of entries copied after a successful call. The re- sulting mib and size may be used in subsequent sysctl() calls to get the data associated with the requested ASCII name. This interface is intended for use by applications that want to repeatedly request the same variable (the sysctl() function runs in about a third the time as the same request made via the sysctlbyname() function). The sysctlnametomib() function is also useful for fetching mib prefixes and then adding a final component. For example, to fetch process information for processes with pid's less than 100: int i, mib[4]; size_t len; struct kinfo_proc kp; /* Fill out the first three components of the mib */ len = 4; sysctlnametomib("kern.proc.pid", mib, &len); /* Fetch and print entries for pid's < 100 */ for (i = 0; i < 100; i++) { mib[3] = i; len = sizeof(kp); if (sysctl(mib, 4, &kp, &len, NULL, 0) == -1) perror("sysctl"); else if (len > 0) printkproc(&kp); } The top level names are defined with a CTL_ prefix in <sys/sysctl.h>, and are as follows. The next and subsequent levels down are found in the in- clude files listed here, and described in separate sections below. Name Next Level Names Description CTL_DEBUG <sys/sysctl.h> Debugging CTL_VFS <sys/mount.h> File system CTL_HW <sys/sysctl.h> Generic CPU, I/O CTL_KERN <sys/sysctl.h> High kernel limits CTL_MACHDEP <sys/sysctl.h> Machine dependent CTL_NET <sys/socket.h> Networking CTL_USER <sys/sysctl.h> User-level CTL_VM <vm/vm_param.h> Virtual memory For example, the following retrieves the maximum number of processes al- lowed in the system: int mib[2], maxproc; size_t len; mib[0] = CTL_KERN; mib[1] = KERN_MAXPROC; len = sizeof(maxproc); sysctl(mib, 2, &maxproc, &len, NULL, 0); To retrieve the standard search path for the system utilities: int mib[2]; size_t len; char *p; mib[0] = CTL_USER; mib[1] = USER_CS_PATH; sysctl(mib, 2, NULL, &len, NULL, 0); p = malloc(len); sysctl(mib, 2, p, &len, NULL, 0); CTL_DEBUG The debugging variables vary from system to system. A debugging variable may be added or deleted without need to recompile sysctl() to know about it. Each time it runs, sysctl() gets the list of debugging variables from the kernel and displays their current values. The system defines twenty (struct ctldebug) variables named debug0 through debug19. They are de- clared as separate variables so that they can be individually initialized at the location of their associated variable. The loader prevents multiple use of the same variable by issuing errors if a variable is initialized in more than one place. For example, to export the variable dospecialcheck as a debugging variable, the following declaration would be used: int dospecialcheck = 1; struct ctldebug debug5 = { "dospecialcheck", &dospecialcheck }; CTL_VFS A distinguished second level name, VFS_GENERIC, is used to get general in- formation about all file systems. One of its third level identifiers is VFS_MAXTYPENUM that gives the highest valid file system type number. Its other third level identifier is VFS_CONF that returns configuration infor- mation about the file system type given as a fourth level identifier (see getvfsbyname(3) as an example of its use). The remaining second level identifiers are the file system type number returned by a statfs(2) call or from VFS_CONF. The third level identifiers available for each file system are given in the header file that defines the mount argument structure for that file system. CTL_HW The string and integer information available for the CTL_HW level is de- tailed below. The changeable column shows whether a process with appropri- ate privilege may change the value. Second Level Name Type Changeable HW_MACHINE string no HW_MODEL string no HW_NCPU integer no HW_BYTEORDER integer no HW_PHYSMEM integer no HW_USERMEM integer no HW_PAGESIZE integer no HW_FLOATINGPT integer no HW_MACHINE_ARCH string no HW_REALMEM integer no HW_AVAILPAGES integer no HW_MACHINE The machine class. HW_MODEL The machine model HW_NCPU The number of cpus. HW_BYTEORDER The byteorder (4321 or 1234). HW_PHYSMEM Amount of physical memory (in bytes), minus the amount used by the kernel, pre-loaded modules, and (on x86) the dcons buffer. HW_USERMEM Amount of memory (in bytes) which is not wired. HW_PAGESIZE The software page size. HW_FLOATINGPT Nonzero if the floating point support is in hardware. HW_MACHINE_ARCH The machine dependent architecture type. HW_REALMEM Amount of memory (in bytes) reported by the firmware. That value is sometimes not sane; in that case, the kernel reports the max memory address instead. HW_AVAILPAGES The same value as HW_PHYSMEM, measured in pages rather than bytes. CTL_KERN The string and integer information available for the CTL_KERN level is de- tailed below. The changeable column shows whether a process with appropri- ate privilege may change the value. The types of data currently available are process information, system vnodes, the open file entries, routing ta- ble entries, virtual memory statistics, load average history, and clock rate information. Second Level Name Type Changeable KERN_ARGMAX integer no KERN_ARND integer no KERN_BOOTFILE string yes KERN_BOOTTIME struct timeval no KERN_CLOCKRATE struct clockinfo no KERN_FILE struct xfile no KERN_HOSTID integer yes KERN_HOSTUUID string yes KERN_HOSTNAME string yes KERN_IOV_MAX integer yes KERN_JOB_CONTROL integer no KERN_LOCKF struct kinfo_lockf no KERN_LOGSIGEXIT integer yes KERN_MAXFILES integer yes KERN_MAXFILESPERPROC integer yes KERN_MAXPHYS integer no KERN_MAXPROC integer no KERN_MAXPROCPERUID integer yes KERN_MAXVNODES integer yes KERN_NGROUPS integer no KERN_NISDOMAINNAME string yes KERN_OSRELDATE integer no KERN_OSRELEASE string no KERN_OSREV integer no KERN_OSTYPE string no KERN_POSIX1 integer no KERN_PROC node not applicable KERN_PS_STRINGS integer no KERN_SAVED_IDS integer no KERN_SECURELVL integer raise only KERN_UPDATEINTERVAL integer no KERN_USRSTACK integer no KERN_VERSION string no KERN_ARGMAX The maximum bytes of argument to execve(2). KERN_ARND arc4rand(9) Fills the buffer with random bytes from in-kernel ran- dom data generator. This is an alternative interface for read(2) of random(4) device, which does not depend on accessibility and correct mounting options of the devfs(4) node. KERN_BOOTFILE The full pathname of the file from which the kernel was loaded. KERN_BOOTTIME A struct timeval structure is returned. This structure contains the time that the system was booted. KERN_CLOCKRATE A struct clockinfo structure is returned. This structure contains the clock, statistics clock and profiling clock frequencies, the number of micro-seconds per hz tick and the skew rate. KERN_FILE Return the entire file table. The returned data consists of an ar- ray of struct xfile, whose size depends on the current number of such objects in the system. KERN_HOSTID Get or set the host ID. KERN_HOSTUUID Get or set the host's universally unique identifier (UUID). KERN_HOSTNAME Get or set the hostname. KERN_IOV_MAX The maximum accepted number of elements in an input-output vector (iovec), see readv(2) and writev(2). KERN_JOB_CONTROL Return 1 if job control is available on this system, otherwise 0. KERN_LOCKF Returns the list of the file advisory locks currently known to ker- nel. KERN_LOGSIGEXIT Controls logging of process exit due to untrapped signals. KERN_MAXFILES The maximum number of files that may be open in the system. KERN_MAXFILESPERPROC The maximum number of files that may be open for a single process. This limit only applies to processes with an effective uid of nonzero at the time of the open request. Files that have already been opened are not affected if the limit or the effective uid is changed. KERN_MAXPHYS Specifies the maximum block I/O size. Can be changed by the tun- able kern.maxphys. KERN_MAXPROC The maximum number of concurrent processes the system will allow. KERN_MAXPROCPERUID The maximum number of concurrent processes the system will allow for a single effective uid. This limit only applies to processes with an effective uid of nonzero at the time of a fork request. Processes that have already been started are not affected if the limit is changed. KERN_MAXVNODES The maximum number of vnodes available on the system. KERN_NGROUPS The maximum number of supplemental groups. KERN_NISDOMAINNAME The name of the current YP/NIS domain. KERN_OSRELDATE The kernel release version in the format MmmRxx, where M is the ma- jor version, mm is the two digit minor version, R is 0 if release branch, otherwise 1, and xx is updated when the available APIs change. The userland release version is available from <osreldate.h>; parse this file if you need to get the release version of the currently installed userland. KERN_OSRELEASE The system release string. KERN_OSREV The system revision string. KERN_OSTYPE The system type string. KERN_POSIX1 The version of IEEE Std 1003.1 ("POSIX.1") with which the system attempts to comply. KERN_PROC Return selected information about specific running processes. For the following names, an array of struct kinfo_proc structures is returned, whose size depends on the current number of such ob- jects in the system. Third Level Name Fourth Level KERN_PROC_ALL None KERN_PROC_PID A process ID KERN_PROC_PGRP A process group KERN_PROC_SESSION A session KERN_PROC_TTY A tty device KERN_PROC_UID effective user ID KERN_PROC_RUID A real user ID KERN_PROC_GID effective group ID KERN_PROC_RGID A real group ID For the following names, the miscellaneous information about the target process, which is specified by the fourth level of the oid name, is returned. A process ID of -1 specifies the current process. Third Level Name Fourth Level KERN_PROC_ARGS Set of strings KERN_PROC_PATHNAME String KERN_PROC_KSTACK struct kinfo_stack [] KERN_PROC_VMMAP struct kinfo_vmentry [] KERN_PROC_FILEDESC struct kinfo_file [] KERN_PROC_GROUPS gid_t [] KERN_PROC_ENV Set of strings KERN_PROC_AUXV Elf_Auxinfo [] KERN_PROC_RLIMIT Integer KERN_PROC_PS_STRINGS Integer KERN_PROC_UMASK Integer/short KERN_PROC_OSREL Integer KERN_PROC_SIGTRAMP Integer KERN_PROC_CWD String KERN_PROC_NFDS Integer KERN_PROC_SIGFASTBLK Integer KERN_PROC_VM_LAYOUT struct kinfo_vm_layout KERN_PROC_RLIMIT_USAGE rlim_t [] KERN_PROC_KQUEUE struct kinfo_knote [] KERN_PROC_ARGS The command line argument array is returned in a flattened form, i.e., zero-terminated arguments follow each other. The total size of array is returned. It is also possible for a process to set its own process title this way. KERN_PROC_PATHNAME The path of the process' text file is returned. KERN_PROC_KSTACK The in-kernel call stacks for the threads of the specified process. KERN_PROC_VMMAP The description of the map entries for the process. Also refer to kinfo_getvmmap(3). KERN_PROC_FILEDESC The file descriptors for files opened in the specified process. Also refer to kinfo_getfile(3). KERN_PROC_GROUPS Groups associated with the process. KERN_PROC_ENV The set of strings representing the environment of the specified process. Note that from the kernel point of view, environment exists only at the time of execve(2) system call. This node method tries to reconstruct the environment from the known breadcrumbs left in the process address space, but it is not guaranteed to succeed or to represent the current value as maintained by the program. KERN_PROC_AUXV The set of ELF auxv entries. See the note above about en- vironment, which is also applicable to auxv. KERN_PROC_RLIMIT Additinal OID name element must be supplied, specifiing the resource name as in getrlimit(2). The call returns the given resource limit for the process. KERN_PROC_PS_STRINGS Returns the location of the ps_strings structure at the time of the last call to execve(2) in the specified process. KERN_PROC_UMASK The current umask value, see umask(2). KERN_PROC_OSREL The value of osrel for the process, that is the osrel the currently executed image was compiled for. Read from the note of the elf executable at execve(2) time. Might be modified by the process. KERN_PROC_SIGTRAMP Address of the signal trampoline in the process address space, where, simplifying, the kernel passes control for signal delivery. KERN_PROC_CWD Returns the current working directory for the process. KERN_PROC_NFDS Returns the total number of opened file descriptors for the process. KERN_PROC_SIGFASTBLK Returns the address of the sigfastblock(2) location, if ac- tive. KERN_PROC_VM_LAYOUT Fills a structure describing process virtual address space layout. KERN_PROC_RLIMIT_USAGE Like KERN_PROC_RLIMIT, but instead of the limit, returns the accounted resource usage. For resources which do not have a meaningful current value, -1 is returned. KERN_PROC_KQUEUE Fills an array of structures describing events registered with the specified kqueue. The next two node's values are the pid and kqfd, the process ID of the process, and the file descriptor of the kqueue in that process, to query. KERN_PS_STRINGS Reports the location of the process ps_strings structure after exec, for the ABI of the querying process. KERN_SAVED_IDS Returns 1 if saved set-group and saved set-user ID is available. KERN_SECURELVL The system security level. This level may be raised by processes with appropriate privilege. It may not be lowered. KERN_USRSTACK Reports the top of the main thread user stack for the current process. KERN_VERSION The system version string. CTL_NET The string and integer information available for the CTL_NET level is de- tailed below. The changeable column shows whether a process with appropri- ate privilege may change the value. Second Level Name Type Changeable PF_ROUTE routing messages no PF_INET IPv4 values yes PF_INET6 IPv6 values yes PF_ROUTE Return the entire routing table or a subset of it. The data is re- turned as a sequence of routing messages (see route(4) for the header file, format and meaning). The length of each message is contained in the message header. The third level name is a protocol number, which is currently al- ways 0. The fourth level name is an address family, which may be set to 0 to select all address families. The fifth, sixth, and seventh level names are as follows: Fifth level Sixth Level Seventh Level NET_RT_FLAGS rtflags None NET_RT_DUMP None None or fib number NET_RT_IFLIST 0 or if_index None NET_RT_IFMALIST 0 or if_index None NET_RT_IFLISTL 0 or if_index None NET_RT_NHOPS None fib number The NET_RT_IFMALIST name returns information about multicast group memberships on all interfaces if 0 is specified, or for the inter- face specified by if_index. The NET_RT_IFLISTL is like NET_RT_IFLIST, just returning message header structs with additional fields allowing the interface to be extended without breaking binary compatibility. The NET_RT_IFLISTL uses 'l' versions of the message header structures: struct if_msghdrl and struct ifa_msghdrl. NET_RT_NHOPS returns all nexthops for specified address family in given fib. PF_INET Get or set various global information about the IPv4 (Internet Pro- tocol version 4). The third level name is the protocol. The fourth level name is the variable name. The currently defined pro- tocols and names are: Protocol Variable Type Changeable icmp bmcastecho integer yes icmp maskrepl integer yes ip forwarding integer yes ip redirect integer yes ip ttl integer yes udp checksum integer yes The variables are as follows: icmp.bmcastecho Returns 1 if an ICMP echo request to a broadcast or multi- cast address is to be answered. icmp.maskrepl Returns 1 if ICMP network mask requests are to be answered. ip.forwarding Returns 1 when IP forwarding is enabled for the host, mean- ing that the host is acting as a router. ip.redirect Returns 1 when ICMP redirects may be sent by the host. This option is ignored unless the host is routing IP pack- ets, and should normally be enabled on all systems. ip.ttl The maximum time-to-live (hop count) value for an IP packet sourced by the system. This value applies to normal trans- port protocols, not to ICMP. udp.checksum Returns 1 when UDP checksums are being computed and checked. Disabling UDP checksums is strongly discouraged. For variables net.inet.*.ipsec, please refer to ipsec(4). PF_INET6 Get or set various global information about the IPv6 (Internet Pro- tocol version 6). The third level name is the protocol. The fourth level name is the variable name. For variables net.inet6.* please refer to inet6(4). For variables net.inet6.*.ipsec6, please refer to ipsec(4). CTL_USER The string and integer information available for the CTL_USER level is de- tailed below. The changeable column shows whether a process with appropri- ate privilege may change the value. Second Level Name Type Changeable USER_BC_BASE_MAX integer no USER_BC_DIM_MAX integer no USER_BC_SCALE_MAX integer no USER_BC_STRING_MAX integer no USER_COLL_WEIGHTS_MAX integer no USER_CS_PATH string no USER_EXPR_NEST_MAX integer no USER_LINE_MAX integer no USER_LOCALBASE string no USER_POSIX2_CHAR_TERM integer no USER_POSIX2_C_BIND integer no USER_POSIX2_C_DEV integer no USER_POSIX2_FORT_DEV integer no USER_POSIX2_FORT_RUN integer no USER_POSIX2_LOCALEDEF integer no USER_POSIX2_SW_DEV integer no USER_POSIX2_UPE integer no USER_POSIX2_VERSION integer no USER_RE_DUP_MAX integer no USER_STREAM_MAX integer no USER_TZNAME_MAX integer no USER_BC_BASE_MAX The maximum ibase/obase values in the bc(1) utility. USER_BC_DIM_MAX The maximum array size in the bc(1) utility. USER_BC_SCALE_MAX The maximum scale value in the bc(1) utility. USER_BC_STRING_MAX The maximum string length in the bc(1) utility. USER_COLL_WEIGHTS_MAX The maximum number of weights that can be assigned to any entry of the LC_COLLATE order keyword in the locale definition file. USER_CS_PATH Return a value for the PATH environment variable that finds all the standard utilities. USER_EXPR_NEST_MAX The maximum number of expressions that can be nested within paren- thesis by the expr(1) utility. USER_LINE_MAX The maximum length in bytes of a text-processing utility's input line. USER_LOCALBASE Return the value of localbase that has been compiled into system utilities that need to have access to resources provided by a port or package. USER_POSIX2_CHAR_TERM Return 1 if the system supports at least one terminal type capable of all operations described in IEEE Std 1003.2 ("POSIX.2"), other- wise 0. USER_POSIX2_C_BIND Return 1 if the system's C-language development facilities support the C-Language Bindings Option, otherwise 0. USER_POSIX2_C_DEV Return 1 if the system supports the C-Language Development Utili- ties Option, otherwise 0. USER_POSIX2_FORT_DEV Return 1 if the system supports the FORTRAN Development Utilities Option, otherwise 0. USER_POSIX2_FORT_RUN Return 1 if the system supports the FORTRAN Runtime Utilities Op- tion, otherwise 0. USER_POSIX2_LOCALEDEF Return 1 if the system supports the creation of locales, otherwise 0. USER_POSIX2_SW_DEV Return 1 if the system supports the Software Development Utilities Option, otherwise 0. USER_POSIX2_UPE Return 1 if the system supports the User Portability Utilities Op- tion, otherwise 0. USER_POSIX2_VERSION The version of IEEE Std 1003.2 ("POSIX.2") with which the system attempts to comply. USER_RE_DUP_MAX The maximum number of repeated occurrences of a regular expression permitted when using interval notation. USER_STREAM_MAX The minimum maximum number of streams that a process may have open at any one time. USER_TZNAME_MAX The minimum maximum number of types supported for the name of a timezone. CTL_VM The string and integer information available for the CTL_VM level is de- tailed below. The changeable column shows whether a process with appropri- ate privilege may change the value. Second Level Name Type Changeable VM_LOADAVG struct loadavg no VM_TOTAL struct vmtotal no VM_SWAPPING_ENABLED integer maybe VM_V_FREE_MIN integer yes VM_V_FREE_RESERVED integer yes VM_V_FREE_TARGET integer yes VM_V_INACTIVE_TARGET integer yes VM_V_PAGEOUT_FREE_MIN integer yes VM_OVERCOMMIT integer yes VM_LOADAVG Return the load average history. The returned data consists of a struct loadavg. VM_TOTAL Return the system wide virtual memory statistics. The returned data consists of a struct vmtotal. VM_SWAPPING_ENABLED 1 if process swapping is enabled or 0 if disabled. This variable is permanently set to 0 if the kernel was built with swapping dis- abled. VM_V_FREE_MIN Minimum amount of memory (cache memory plus free memory) required to be available before a process waiting on memory will be awak- ened. VM_V_FREE_RESERVED Processes will awaken the pageout daemon and wait for memory if the number of free and cached pages drops below this value. VM_V_FREE_TARGET The total amount of free memory (including cache memory) that the pageout daemon tries to maintain. VM_V_INACTIVE_TARGET The desired number of inactive pages that the pageout daemon should achieve when it runs. Inactive pages can be quickly inserted into process address space when needed. VM_V_PAGEOUT_FREE_MIN If the amount of free and cache memory falls below this value, the pageout daemon will enter "memory conserving mode" to avoid dead- lock. VM_OVERCOMMIT Overcommit behaviour, as described in tuning(7). RETURN VALUES Upon successful completion, the value 0 is returned; otherwise the value -1 is returned and the global variable errno is set to indicate the error. FILES <sys/sysctl.h> definitions for top level identifiers, second level kernel and hardware identifiers, and user level iden- tifiers <sys/socket.h> definitions for second level network identifiers <sys/gmon.h> definitions for third level profiling identifiers <vm/vm_param.h> definitions for second level virtual memory identi- fiers <netinet/in.h> definitions for third level IPv4/IPv6 identifiers and fourth level IPv4/v6 identifiers <netinet/icmp_var.h> definitions for fourth level ICMP identifiers <netinet/icmp6.h> definitions for fourth level ICMPv6 identifiers <netinet/udp_var.h> definitions for fourth level UDP identifiers ERRORS The following errors may be reported: [EFAULT] The buffer name, oldp, newp, or length pointer oldlenp contains an invalid address. [EINVAL] The name array is less than two or greater than CTL_MAX- NAME. [EINVAL] A non-null newp is given and its specified length in newlen is too large or too small. [ENOMEM] The length pointed to by oldlenp is too short to hold the requested value. [ENOMEM] The smaller of either the length pointed to by oldlenp or the estimated size of the returned data exceeds the system limit on locked memory. [ENOMEM] Locking the buffer oldp, or a portion of the buffer if the estimated size of the data to be returned is smaller, would cause the process to exceed its per- process locked memory limit. [ENOTDIR] The name array specifies an intermediate rather than terminal name. [EISDIR] The name array specifies a terminal name, but the actual name is not terminal. [ENOENT] The name array specifies a value that is unknown. [EPERM] An attempt is made to set a read-only value. [EPERM] A process without appropriate privilege attempts to set a value. SEE ALSO confstr(3), kinfo_getproc(3), kvm(3), sysconf(3), sysctl(8) HISTORY The sysctl() function first appeared in 4.4BSD. FreeBSD ports 15.1 July 31, 2025 SYSCTL(3)
NAME | LIBRARY | SYNOPSIS | DESCRIPTION | RETURN VALUES | FILES | ERRORS | SEE ALSO | HISTORY
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